This is Thomas Mayer's blog about vacuum tube audio, to share updates about new amplifiers and preamplifiers and ELROG vacuum tubes.

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Wednesday, January 22, 2014

ShellacSavor, Part 1 : Introduction

Hi!

After last years European Triode Festival, I stayed another day in Berlin. I visited my good friend Frank W. who introduced me to Shellac records. I never seriously listened to shellacs before so this was a first for me and it was a big surprise. Frank uses a Octal Preamplifier Mk1 and a 801A power amp which I built. His speakers are an enhanced version of the Rho from David Haigner. For Shellacs Frank uses a mono phono preamp.

Shellac recordings reproduced over a good system and with a suitable preamp which matches the recording EQ curves properly (more about this later) are very special. The reproduction has not much in common with todays expectations towards HiFi. No deep bass, no extended treble, not much resolution and horrible surface noise. But that midrange is pure magic. The emotional impact is simply overwhelming. Voices not only go under your skin, they touch your soul.

Being mono, Shellacs don't reproduce a soundstage between left and right, if played through a regular stereo system. But they can give a beautiful impression of depth and space which is more natural than the artificial space reproduced by many stereo records.

We spent the whole evening listening to a lot of Shellacs. Mostly Jazz records from the era between World War II and the introduction of vinyl. Frank characterised the sound of the Shellacs nicely in his typical Berlin accent : 'dat jeht direkt auf de Pumpe, wa?'. Translated this means: 'this goes straight to the heart', but this does not get the twist of his Berlin slang, a more appropriate translation might be : 'this goes straight on da pump, huh?'.

I have heard and built a lot of systems and listend to different audio media during the last decades. It is difficult to impress me with something. But what Frank showed me opened a whole new world for me to explore. Of course this does not mean that I will switch to Shellac entirely. I will also not change the name of the blog to ShellacSavor. Vinyl will continue to be my main source and I am also not advert to digital. Shellac is a beautiful addition for a different kind of music reproduction. Thanks a lot to Frank for introducing me to the world of Shellac.

Frank had been asking me to build a dedicated phono preamp with some more EQ curves than the unit he has since a while. Now after I heard this it was clear for me that I have to design a proper phono stage with as many EQ settings as possible.

I already built a Phono preamp with variable EQ curves in 2011. But that preamp mostly only covered EQ settings for vinyl mono records, not for shellacs. So I started with researching the EQ settings used by various recording studios for their shellac pressings.

The basic shape of the equalisation needed is similar to that of vinyl records, it only varies in the corner frequencies. This is how a generic EQ in the reproduction looks like:

At a certain frequency, which defines the 'rumble shelf', the response starts to roll off. Attenuation then stops at the Turnover frequency and stays flat until the treble roll off point after which the response rolls off again. The Rumble shelf can extend up to 100Hz. For RIAA it is 50Hz. Some shellacs have been recorded without a rumble shelf. But still that frequency had to be set somewhere since the amplitude would otherwise rise to infinity when moving towards 0Hz. Typically that was around 15-20Hz.

Rumble shelf and Turnover frequency have usually been named by a 3 digit number, followed by a letter. The 3 digit number would indicate the turnover frequency. For different rumble shelfs letters have been assigned.

It is apparent that there was a great variety in recording curves. Rumble shelf frequency spreading from 15Hz up to 100Hz and turnover frequencies spanning 2 Octaves from 200Hz up to 800Hz. That translates to differences of up two 12dB in the frequency region where the ear is the most sensitive. For serious shellac reproduction an adjustable equalisation is mandatory.

There was an equally wide spread in the high frequency EQ. This was not defined by the corner frequency at which the roll off starts but rather by the roll off in dB at 10kHz. I found indications of 12 different high frequency equalisation settings:

Again a huge spread. A difference of 18dB at 10kHz for different records. Here it is even more apparent that a shellac would sound very wrong if the treble EQ is not done right. It could either be totally dull or annoyingly shrieking.

The EQ curve used for recording is specified by a combination of both, for example:

Many different combinations had been in use. In order to implement a preamp which can reproduce all of them correctly, it makes sense to use two separate selector switches for rumble shelf/turnover and treble roll off. This way all the settings can be implemented with practical 12 position switches.
Of course such an implementation will allow combinations which had never been used, so the setting should be done according to a list as the one mentioned above. An advantage of such a dual switch set up is the possibility to also fine tune the sound and deliberately use a 'wrong' setting as a tone control. For example a roll off setting with less attenuation could be used to boost high frequencies a bit for dull sounding records or to attenuate it if it is too bright sounding.

Here is a sketch of the faceplate for the variable EQ preamplifier:

The circuit development and tube selection are underway. Stay tuned for updates about Shellac reproduction.

6 comments:

what a coincidence! Two months ago I discovered Shellacs as well. Actually it came along with my new HMV 102 portable gramophone. The reproduction of music of a pure mechanical system was overwhelming when I first listened to it. Even though a steel-needle isn't the best way to treat your records, it works the best with silent pre WWII recordings. But the whole magic of the midrange, as you said it, actually comes with the later recordings like Ella Fitzgeralds and Louis Armstrongs "Dream a little Dream" from 1950.I can't wait to hear it on a good system with an electronic pick-up.